1. Technical Field
The invention relates generally to an Asynchronous Transfer Mode (ATM) network wherein a connection is established between a source ATM device and a destination ATM device by the intermediary of a plurality of network switching nodes, and relates in particular to a method of returning loopback cells from the connection output adapter in one of the switching nodes located on the route used by the connection.
2. Description of the Related Art
The use of ATM switching nodes in an IP network has become one of the most attractive solutions since ATM hardware switches have been extensively studied and are widely available in many different architectures.
When a connection is established in an ATM network from a source ATM device to a destination ATM device by the intermediary of a plurality of switching nodes, the incoming cells to a switching node are automatically routed to the next switching node of the connection. For this, each cell is composed of an ATM header and of a payload. The ATM protocol engine of the switching node identifies the incoming ATM cell using a lookup table. In the case of a valid cell (“valid” meaning belonging to an existing connection), the protocol engine performs traffic management function (traffic policing, congestion management, priority management) and queues the cell in an appropriate queue. A scheduler using priority based scheduling procedures selects queues from which cells are to be transmitted. Those cells are removed from their queue. Prior to transmission, the protocol engine adds the following routing labels to the cell: the switch routing label (SRL) and the protocol engine correlator (PEC). The resulting internal cell format used within the switching node will be denominated hereafter as a “labeled” cell. The SRL contains either explicitly the destination blade or a pointer to a translation table located in the switching device and containing the explicit destination blade. The PEC is a pointer used by the protocol engine of the output adapter to identify the connection. The protocol engine in the output destination adapter receives the cell from the switching device. Similarly to the input protocol engine, it identifies the incoming cell by performing a lookup function on the appended protocol engine correlator, runs traffic management functions, queues the cell in the appropriate queues, removes the cell under control of a scheduler from the appropriate queue, removes the appended labels, swaps the ATM label and transmits the cell on the connection destination ATM port(s).
The ATM standards have defined Operation And Maintenance (OAM) procedures. These procedures are based on particular cells identified as OAM cells by means of particular values of the payload type indicator (PTI) field of the ATM cell header. Some of the OAM cells are called loopback cells. They can be either segment or end to end loopback cells and may possibly contain in their payload a source and a destination address indication. The ITU-T I610 specifications define the procedures to be performed by a network equipment when receiving OAM loopback cells. In particular, they define two loopback locations for a switching node, the input adapter and the output adapter, and the loopback condition algorithm using the cells parameters (source address, destination address, segment or end to end). These procedures, by allowing to loopback cells on a connection path at various locations (input or output adapter of the various switching nodes on the connection path) are used either to monitor the connection or for problem determination and failure isolation in the case of a failing connection.
When OAM cells such as loopback cells are received in a switching node of a connection, a classical way to process those cells would be to transmit the cells to a dedicated resource such as a local processor that would perform the OAM procedures. But, such a solution is not appropriate and is expensive inasmuch as it requires the incorporation of microprocessors on the adapter card of the switching node whereas the normal connection cells use ASIC modules which are data processing units specifically designed for the routing of the ATM cells.